The present invention generally relates to the production of single crystal silicon, and specifically, to a process for preparing a molten silicon melt from polycrystalline silicon. The invention particularly relates, in a preferred embodiment, to a process for preparing a molten silicon melt from a mixed charge of chunk and granular polycrystalline silicon.
Most single crystal silicon used for microelectronic circuit fabrication is prepared by the Czochralski (CZ) process. In this process, a single crystal silicon ingot is produced by melting polycrystalline silicon in a crucible, dipping a seed crystal into the molten silicon, withdrawing the seed crystal in a manner sufficient to achieve the diameter desired for the ingot and growing the single crystal at that diameter. The polycrystalline silicon melted to form the molten silicon is typically irregularly shaped chunk polycrystalline silicon prepared by the Siemens process or, alternatively, free-flowing, generally spherically-shaped granular polycrystalline silicon, typically prepared by a fluidized-bed reaction process. The preparation and characteristics of chunk and granular polycrystalline silicon are detailed in F. Shimura, Semiconductor Silicon Crystal Technology, pages 116-121, Academic Press (San Diego Calif., 1989) and the references cited therein.
The initial charging of chunk type polycrystalline silicon into the crucible and the melting thereof can introduce undesirable impurities and defects into the single crystal silicon ingot. For example, when a crucible is initially charged entirely with chunk polycrystalline silicon, the edges of the chunks under the load of a full charge can scratch and gouge the crucible wall, resulting in a damaged crucible and in particles of crucible floating on or being suspended in the silicon melt. These impurities significantly increase the likelihood of dislocations forming within the single crystal, and decrease the dislocation-free single crystal production yields and throughput. Careful arrangement of the chunk-polycrystalline silicon during the initial loading can minimize the thermal stresses. As melting proceeds, however, the charge can shift or the lower portion of the chunk-polycrystalline silicon can melt away and leave either a "hanger" of unmelted material stuck to the crucible wall above the melt or a "bridge" of unmelted material bridging between opposing sides of the crucible wall over the melt. When the charge shifts or a hanger or bridge collapses, it may splatter molten silicon and/or cause mechanical stress damage to the crucible. Additionally, initial loadings of 100% chunk-polycrystalline silicon limits the volume of material which can be charged due to the poor packing densities of such chunk materials. The volume limitations directly impact single crystal throughput.
Problems similarly exist when a CZ crucible is initially charged entirely with granular polycrystalline silicon. Large amounts of power are required to melt the granular polycrystalline due to its low thermal conductivity. The thermal stress induced in the crucible by exposure to such high meltdown-power can cause distortion of the crucible and particles of the crucible to be loosened and suspended in the melt. Like the mechanical stresses, these thermal stresses result in reduced zero-defect crystal production yields and throughput. Other problems associated with initial charges comprising 100% granular polycrystalline silicon are disclosed below with respect to the present invention. Finally, although initial loadings of granular polycrystalline silicon may be volumetrically larger than that of 100% chunk polycrystalline silicon, they typically do not result in higher overall throughput, because the degree of thermal stress on the crucible increases with the size of initial loading.
Whether the crucible is initially loaded with chunk or granular polycrystalline silicon, in many processes it is desired to add polycrystalline silicon to the melt with a feeding/metering system to increase the quantity of molten silicon. The use of such additional loadings of charge-up polycrystalline silicon is known for batch, semi-continuous or continuous process systems. In the batch system, for example, additional silicon may be loaded into the existing melt to achieve full crucible capacity in light of the decrease in volume after the initial polycrystalline silicon charge is melted. Japanese Utility Model Application No. 50-11788 (1975) is exemplary. In semi-continuous and continuous CZ systems, additional polycrystalline silicon is charged to the silicon melt to replenish the silicon withdrawn as the single crystal. F. Shimura, Semiconductor Silicon Crystal Technology, p. 175-83, Academic Press (San Diego Calif., 1989).
Although granular polycrystalline silicon is generally the material of choice to replenish batch, semi-continuous and continuous CZ systems because of its free-flowing form, it is not without its disadvantages. As disclosed by Kajimoto et al. in U.S. Pat. No. 5,037,503, granular polycrystalline silicon prepared by the silane process contains hydrogen in an amount sufficient to cause the silicon granules to burst or explode when they are immersed in molten silicon. The explosion or bursting of the polycrystalline silicon granules causes scattered silicon droplets to accumulate on the surface of the crucible and other components in the crystal puller which can fall into the molten silicon and interrupt crystal growth. As a solution to this problem, Kajimoto et al. suggest reducing the hydrogen content of the granular polycrystalline silicon by preheating the granular polycrystalline in an inert gas atmosphere in a separate heating apparatus until the concentration of H.sub.2 is 7.5 ppm by wt. (210 ppma) or less. While this approach tends to reduce the force with which the granules explode, it does not eliminate the phenomena. Rather, the bursting phenomena can still be experienced with granular polycrystalline having a hydrogen concentration of less than 1 ppm by wt. (28 ppma). To date, granular polycrystalline silicon is available in commercial quantities having a hydrogen concentration ranging from about 0.4 to about 0.7 ppm by wt. (11-20 ppma).